1. Field of the Invention
The present invention generally concerns methods and apparatus for the manufacture of thin-walled structures of thermoplastic material, and is more particularly directed at a centrifugal spray molding method and apparatus for easily, quickly and reliably making relatively thin-walled hollow containers of uniform wall thickness useful as liners for liquid storage tanks. Of particular interest to the present invention is the manufacture of inert liners for aircraft fuel tanks wherein it is important to maintain a uniform thickness of the liner wall to avoid blistering or deformation of the liner under internal pressure.
2. State of the Prior Art
Present methods for manufacturing thin-walled hollow structures of thermoplastic materials for the above stated purposes include rotational molding methods using a mold of cylindrical shape and closed at both ends. The mold is heated to a predetermined temperature and a load of powdered thermoplastic material is introduced into the mold while the mold is rotating simultaneously about its long axis and about a transverse axis. The powdered thermoplastic material is dispersed over the internal cylindrical walls of the rotating mold and melts thereon due to the elevated temperature of the mold surface. The thermoplastic material thus adheres to the mold surface and forms a sheet which takes on the shape of the internal mold surface. It has been found, however, that the wall thickness of fuel tank liners obtained by rotational molding methods cannot be kept within the desired tolerances. Little control can be maintained over the dispersion of the thermoplastic material because such dispersion is essentially random within the rotating mold, and the full quantity of thermoplastic material necessary for the manufacture of the desired liner is introduced at one time into the container for subsequent distribution over the mold surfaces by centrifugal forces provided by the prior art apparatus.
The applicant is also aware of the following patents relating to centrifugal casting and molding methods and devices which are basically centered around a revolving drum and apparatus for delivering a moldable material to the rotating interior surface of the drum.
U.S. Pat. No. 1,840,027 to Fetter, 1/5/32 PA1 U.S. Pat. No. 4,150,176 to Beckers, 4/17/79 PA1 U.S. Pat. Re. No. 25,587 to Wiltshire, 6/2/64 PA1 U.S. Pat. No. 3,666,528 to Barnhardt, 5/30/72 PA1 U.S. Pat. No. 3,150,219 to Schmidt, 9/22/64 PA1 U.S. Pat. No. 1,223,602 to Moir et al, 4/24/17 PA1 U.S. Pat. No. 1,142,829 to Maywald, 6/15/15
The Fetter patent discloses a method for making inner tubes of liquid rubber by spraying liquid rubber onto the inner cylindrical surface of a rotating tubular mold which is heated to a temperature sufficient to cause curing and hardening of the liquid rubber. The solidified material is then removed from the tubular mold. An axially movable nozzle is reciprocated between the two ends of the rotating cylindrical mold to coat the cylindrical surface with the liquid rubber material.
The remaining patents similarly deal with methods and apparatus for coating a rotating inner cylindrical surface with liquid material, either alone or in admixture with filamentary reinforcing material. Heat is applied either concurrently with the application of the liquid material or subsequently thereto to induce curing and solidification of the applied liquid.